Method for producing motion and force by controlling the twin structure orientation of a material and its uses

ABSTRACT

The present invention refers to a method and/or force in a material having a twinned structure. According to the method, a sufficiently high external magnetic field applied to the material reorients a the twin structure thereby producing motion/force. The operation is possible if the magnetocrystalline anisotropy energy is higher than or comparable to the energy of the reorientation of the twin structure to produce a certain strain.

FIELD OF THE INVENTION

[0001] The present invention relates to a method for controlling thetwin orientation by the magnetic field in a material having such astructure. The aim is to produce shape changes, motion and force byusing actuators based on this method.

BACKGROUND OF THE INVENTION

[0002] Control of motion and force is one of the basic elements inmechanical engineering. Development of new materials has made itpossible to produce motion and force using special functional materialscalled actuator materials. The most important groups of actuatormaterials available are piezoelectric ceramics, magnetostrictiveintermetallics, and shape memory alloys. Piezoelectric ceramics developstrains when subjected to an electric field. Frequency response of thesematerials is fast, but the strain amplitudes are very small, whichlimits their applicability. Magnetostrictive materials are strained whena magnetic field is imposed on them. Certain high-magnetostrictiveintermetallics (e.g., Terfenol-D, Etrema Products, Inc., Ames, Iowa,USA) offer strains up to 0.17%, which is an order of magnitude higherthan those of the current piezolectrics. The frequency response of themagnetostrictive intermetallics is lower than that of thepiezoelectrics.

[0003] Shape memory metals are materials which, when plasticallydeformed at one temperature, can recover their original undeformed stateupon raising their temperature above an alloy-specific transformationtemperature. In these materials, crystal structure undergoes a phasetransformation into, and out of, a martensite phase when subjected tomechanical loads or temperature. The process when a mechanicallydeformed shape memory material returns to its original form afterheating is called a one-way shape memory effect. Cooling the materialsubsequently will not reverse the shape change. The one-way shape memoryeffect is utilized in fastening, tightening and prestressing devices.Strains of several percent can be completely recovered, and recoverystresses of over 900 MPa have been attained. In the case of the two-wayeffect, no deformation is required, and the material “remembers” twoconfigurations that are obtained by heating and cooling toalloy-specific temperatures. The temperature difference between the twoconfigurations can be as small as 1 to 2 K Materials that exhibit atwo-way shape memory effect are used to develop forces and displacementsin actuators. Those actuators are applied in machinery, robotics andbiomedical engineering. The most extensively used shape memory materialsare Ni—Ti and Cu-based alloys. A drawback of the shape memory actuatorsis their slow response due to the thermal control (especially incooling) and low efficiency (energy conversion), which in many alloys isonly about one percent.

[0004] In order for the shape memory effect to occur, the material mustexhibit a twinned substructure. The shape change of the shape memorymaterial is based on the reorientation of the twin structure in theexternal stress field. A two-dimensional illustration of the Winreorientation is presented in FIG. 1. FIG. 1(a) shows two twin variants,denoted by 1 and 2, with equal proportions in the absence of theexternal stress. When the stress is applied, FIG. 1(b), the twinboundaries move and variant 2 grows at the expense of variant 1,producing the shape which better accommodates the applied stress. Theresult of moving a twin boundary is thus to convert one twin variantinto another. The variants which are most favorably oriented to theapplied stress will grow. Ultimately, a single variant of martensite canbe produced by straining a sufficient amount, as illustrated in FIG.1(c). In the martensite phase, the variants are usually oriented inseveral crystallographic directions. Therefore, complex shape changes ofthe material can be produced by the reorientation of the twin structure,and a full shape recovery will be obtained. Crystallographic analysishas shown that the boundaries between the martensite plates also behaveas twin boundaries, i.e., the individual plates of martensite themselvesare twins with respect to adjoining plates. Thus the term “twinboundaries”, generally refers to the boundaries between martensiteplates as well as the boundaries between the boundaries within theplates (this definition also concerns the magnetically controlled twinboundaries discussed below). In some materials, applied stress inducesformation of the martensite phase whose twinned substructure ispreferentially oriented according to the applied stress.

[0005] Reorientation of the twin structure is responsible for therecoverable strains of several percent in appropriate materials (e.g.,close to 10 percent in Ni—Ti shape memory alloys). In some alloys thestress required to reorient the twin structure is very low. FIG. 2 showsthe stress-strain curves for the selected shape memory materials. It isseen that strains of 4 percent are attained by stresses of 20 to 50 MPain most of those alloys. Stresses as low as 1 to 30 MPa result instrains of one percent. Strain energy densities needed to produce thestrain of 1 percent by the reorientation of the twin variants are theareas restricted by the stress-strain curves, strain axis and thevertical dashed line in FIG. 2. The strain energy densities for thealloys In—TI, Ni—Mn—Ga (ferromagnetic Ni₂MnGa), CuZn—Sn and Cu—Zn are10⁴, 8.5×10⁴, 1.1×10⁵ and 2.3×10⁵ J/m³, respectively.

[0006] In the following, magnetic anisotropy energy is introduced,because it plays an important role in the present invention. Inferromagnetic crystals magnetocrystalline anisotropy energy is an energywhich directs the magnetization along certain definite crystallographicaxes called directions of easy magnetization. FIG. 3 shows themagnetization culves of single crystalline cobalt which has a hexagonalcrystal structure. Its easy direction of magnetization is the parallelaxis of the unit cell. The saturation is reached at a low magnetic fieldvalue in this direction, as shown in FIG. 3. Saturating the sample inthe basal plane is much more difficult. A magnetic field over 8000 Oe isneeded for saturation. The basal plane direction is called a harddirection of magnetization. Magnetic anisotropy energy densitycorresponding to the magnetization processes in different directions isthe area between the magnetization curves for those directions. Incobalt the energy density needed to saturate the sample in the harddirection is about 5×10⁵ J/m³ (the area between the saturation curves inFIG. 3). Anisotropy energy densities of magnetically hard Fe— andCo-based alloys range from 10⁵ up to 10⁷ J/m³. The highest anisotropyenergy densities (K1 values), close to 10⁸ J/m³, are in 4f metals at lowtemperatures. In intermetallic compounds such as Co₅Nd, Fe₁₄Nd₂B andSm₂Co₁₇ the anisotropy energy densities at room temperature are 1.5×10⁷,5×10⁷ and 3.2×10⁶ J/m³, respectively.

SUMMARY OF THE INVENTION

[0007] This invention concerns an operational principle of themagnetically driven actuators that produce motion and force. Theoperation is based on the magnetic-field controlled reorientation of thetwin structure of the actuator material. These kinds of actuators canproduce strains of several percent (as large as the shape memorymaterials produce). Because of the magnetic control of the newactuators, the response times are much faster, control more precise, andefficiency better than those of the shape memory materials. The newmagnetically driven actuators will exhibit a great potential inmechanical engineering. They will replace hydraulic, pneumatic andelectromagnetic drives in many applications. Employment of theseactuators leads to simpler, lighter, and more reliable constructionsthan use of conventional technology. Because the twin reorientationoccurs in three dimensions, complex shape changes can be produced underthe magnetic control. Applicability of this invention is expanded by thepossibility for controlling and supplying the power of the actuators ata distance. The whole machine developing a controlled motion or desiredshape changes (e.g., bending, twisting, clipping, fastening, pumpingfluids) may be a small appropriately shaped and preoriented piece ofmaterial. Due to the small twin size in many materials, this inventionis expected to have great potential also in micro- and nanotechnology.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] FIGS. 1(a) to 1(c) show a schematic (two dimensional)presentation of the shape change in martensite material as describedabove, namely turning of the twin variants by stress.

[0009]FIG. 2 shows stress-strain (tensile) curves for single crystallinealloys In—TI, Cu—ZnSn and a Ni—Mn—Ga Heusler-alloy (Ni₂MnGa) and for apolycrystalline Cu—Zn shape memory alloy during the reorientation of thetwin structure.

[0010]FIG. 3 presents magnetization curves of single crystalline cobalt.

[0011]FIG. 4 shows the principle of the present invention, namelyturning of the twin variants by the external magnetic field.

[0012]FIG. 4(a) presents the starting situation in the absence of theexternal magnetic field;

[0013]FIG. 4(b) shows the turning of the twin variant by the appliedmagnetic field H.

[0014] FIGS. 5(a) to 5(c) show the principle of themagnetic-field-induced shape change of the twinned material whichresults in the shape change of the material and the motion and force ofthe actuator, namely;

[0015]FIG. 5(a) presents the starting situation in the absence of theexternal magnetic field;

[0016]FIG. 5(b) shows the step where the external magnetic field H₁ actson the material;

[0017]FIG. 5(c) presents the ultimate situation after the completereorientation of the twin structure by the magnetic field.

[0018]FIG. 6 shows the experimental setup for studying the reorientationof the twin structure by the magnetic field.

DETAILED DESCRIPTION OF THE INVENTION

[0019] This invention is a new method for producing shape changes,motion and/or force in materials, based on the reorientation of the twinstructure under the application of the external magnetic field.

[0020] The present invention is described on the following pages byexplaining the relevant properties of the present invention and byreferring to some figures describing the background for easierunderstanding of the present invention. Reference is made to all FIGS. 2to 6.

[0021]FIG. 4 shows a two-dimensional illustration of the principle ofthe reorientation of the twin structure by the applied magnetic field.In crystalline ferromagnetic materials, magnetization vectors lie alongdirections of easy magnetization in the absence of the external magneticfield. This situation is shown in FIG. 4(a) for two twin variants. Theeasy direction of magnetization is parallel with the side of the unitcells of each variant. It is emphasized that the easy direction is notnecessarily parallel with the side of the unit cell but it can also beany other crystallographic direction characteristic of the material.

[0022] When an external magnetic field is applied on a crystallineferromagnetic material, the magnetization vectors tend to turn from theeasy direction of the unit cell to the direction of the externalmagnetic field. If the magnetocrystalline anisotropy energy, denoted byU_(k) in this presentation, is high, the magnetic field strengthsrequired to turn the magnetization off from the easy directions are alsohigh. This was illustrated for hexagonal cobalt in FIG. 3. If the energyof turning the twin variants, (i.e., the energy of the motion of thetwin boundaries) is low enough compared to the magnetocrystallineanisotropy energy U_(k), the twin variants are turned by the externalmagnetic field, and the magnetization remains in the original easydirection of the turned unit cells. FIG. 4b shows how the unit cells ofone variant are turned into another by the external magnetic field. As aresult, twins in favorable orientation to the magnetic field grow at theexpense of the other twins, as shown in FIG. 5.

[0023]FIG. 5(a) represents the starting situation in the absence of thefield when the twin variants with equal proportions are present.Magnetization is aligned parallel to one side of the unit cell in eachvariant In the figure, only a part of the magnetization vectors isshown. in this illustration twins are assumed to be consisted of onlysingle ferromagnetic domains (recent TEM studies have revealed thattwins in some ferromagnetic martensites e.g Fe—Pt can be consisted oftwo magnetic domains whose domain wall crosses the twin.)

[0024]FIG. 5(b) shows how the unit cells whose easy direction ofmagnetization are different from the direction of the external magneticfield are turned so as to line up with the field direction. This resultsin the growth of the favorably oriented twin variant and the decrease ofthe other variant. Ultimately, only one twin variant may remain, asshown in FIG. 5(c).

[0025] The reorientation of the twin structure described above resultsin the shape change of the material, which can produce motion and forcein the magnetically controlled actuators made from this material. It ispossible also to produce complex shape changes because the reorientationof the twin structure occurs in three dimensions. The originaldimensions of the actuator material may be restored by eliminating thefield, or by turning the field to another direction. Effects of theexternal magnetic field on the orientation of the martensite unit cellscan cause the directed motion of martensite-martensite andaustenite-martensite interfaces, which may also be utilized inactuators. In that case the preferentially oriented twinned martensitegrows at the expense of the parent phase. This growth can also bereversible.

[0026] The magnetic-field-control of the reorientation of the twinvariants is expected to produce recoverable strains of several percentin appropriate materials (analogous to stress-induced recoverablestrains in the shape memory alloys). To reach a certain magneticallyinduced strain, it is necessary that the magnetocrystalline anisotropyenergy U_(k) of the material is larger than or comparable to the energyneeded to reorient the twin variants to achieve this strain. The latterenergy, defined as the energy of the reorientation of the twinstructure, and denoted by E_(tW) includes also strain and dissipationenergy terms related to the shape change of the material. In theactuator applications, U_(k) must be greater than the sum of E_(tW), andthe work of the actuator. The work term may be positive or negative. Ifthe work is negative, the external stress may assist the reorientationof the twin structure and decrease the magnetic field energy required.For the actuator to be able to operate, it is necessary that themagnetic field energy that controls the actuator must be larger than thesum of E_(tW), and the work of the actuator. The higher the U_(k) is,the larger the magnetic field energies are which can be converted tomechanical work of the actuators and, hence, the higher forces areattained.

[0027] In the following, the magnitudes of the anisotropy energies willbe compared with the energies of the reorientation E_(tW) in differentmaterials. As it was shown in FIG. 2, energy densities E_(tW) forproducing strain of 1 percent in the selected martensitic shape memoryalloys are between 10⁴ and 2.3×10⁵ J/m³. On the other hand, there is adiversity of materials available in which magnetic anisotropy energydensities are 10⁵ to 10⁸ J/m³. Some examples (Co—, Fe— andrare-earth-based alloys) were given above. Anisotropy energy densitiesof some materials are even four orders of magnitude larger than theenergy densities E_(tW) for the reorientation of the twin structure,e.g., in In—TI. This large difference in energies U_(k) and E_(tW),reveals that there is a great potential for finding optimal materialsthat combine high anisotropy energy and low E_(tW).

[0028] In some ferromagnetic martensites twin boundaries are highlymobile under application of stress. It was shown in FIG. 2 for theferromagnetic martensitic Ni₂MnGa (single crystalline) that stresses aslow as 10 to 20 MPa in direction [100] cause the reorientation of thetwin variants, resulting in the recoverable strains of 4 percent Toreach a strain of 1 percent in this alloy by the magnetic-field inducedreorientation of the twin structure, anisotropy energy must be largerthan the energy of the reorientation of the twin variants E_(tW),8.5×10⁴ J/m³, according to FIG. 2. This value is quite low and,therefore, the magnetically induced strains are expected to be possiblein this material. In most ferromagnetic shape memory alloys available todate and other iron-based alloys that exhibit a twinned substructure,stresses for aligning the twins are higher, even above 100 MPa. However,their magnetocrystalline anisotropy energies are often high enough forproducing magnetic-field-induced strains based the reorientation of thetwin structure, which has been experimentally demonstrated in somealloys. For example, in a material in which the stress of 100 MPa wouldbe needed to reorient the twin structure producing a strain of 1percent, E_(tW) is calculated to be 5×10⁵ J/m³ (assuming that stressincreases linearly with strain). In order to produce the same strain bythe magnetic-field-induced reorientation of the twin structure,anisotropy energy must be larger than or equal to 5×10⁵ J/m³. Thisanisotropy energy value is the same as that of cobalt and is attainablein many Fe— and Co-based alloys.

[0029] As a third example, let us assume that a very high stress of 500MPa would be needed to produce a strain of 1 percent by thereorientation of the twin structure in some materials. To produce thestrain of the same amount by the magnetic field, anisotropy energydensity of 2.5×10⁶ J/m³ is needed. This anisotropy energy value isattainable in suitable alloys, because the highest anisotropy energiesat room temperature are even 20 times larger. It is emphasized that onlyestimations obtained from some material classes are used in thispresentation for evaluating the magnetocrystalline anisotropy energies,because the anisotropy energy values for the twinned materials with lowE_(tW) cannot be measured using saturation magnetization measurements(see FIG. 3). The reason is that the magnetization does not turn in theapplied field to the hard direction of the unit cells, but thesaturation is reached by turning the twin variants (together with themagnetization vectors) at lower magnetic field levels. The magnetizationmeasurements should be made on single variant samples which are in manycases not possible to produce.

[0030] The development is focused in finding new ferromagnetic materialswhich exhibit high anisotropy energy and low E_(tW). The best materialsmay combine high anisotropy energies coming from the rare earth metalsand highly glissile twin boundaries of a suitable twinned phase. AlsoCo— and Fe-based shape memory materials in which the martensitic latticeis close-packed hexagonal or cubic are promising and are beingdeveloped. The role of the interstitial atoms, especially nitrogen maybe important, because they often increase the anisotropy energy andstrengthen the alloy mechanically, which favors twinning as adeformation mechanism and prevents permanent slip. One interesting groupof the magnetically controlled actuator materials are Heusler alloys(e.g., Ni₂MnGa-type) in which Mn is responsible for their ferromagneticproperties.

[0031] The velocity of the twin boundaries is very fast in manymaterials, even a fraction of the speed of sound. This means that themagnetic-field-induced strokes are very fast in suitable actuatormaterials, and the actuators can operate at high frequencies.

EXAMPLES

[0032] The reorientation of the twin structure by the magnetic field wasexperimentally studied in Fe—Ni—Co—Ti, Fe—Ni—C and Fe—Mn—N -basedalloys. These materials are ferromagnetic and exhibit a twinnedmicrostructure. The anisotropy energies were measured to be typicallyabout 5×10⁵ J/m³ for Fe—Ni—Co—Ti alloys and 2×10⁵ J/m³ for Fe—Ni—Calloys. These are expected to be sufficiently high for producing themagnetic-field-induced strains based on the reorientation of the twinstructure. The experimental setup employed in the present studies andexamples of the measurements are shown in the following.

[0033] The Experimental Setup

[0034] The principle of the equipment for studying the effects of stressand magnetic field on the twin structure is shown in FIG. 6. Thisequipment makes it possible to apply axial and torsional stresses on thesample, and to measure the corresponding strains. The sample 6 was fixedin two coaxial supporting tubes 1 and 2. Tube 1 was fixed and tube 2 wasused for straining the sample. The sample chamber was surrounded by acoil 7 for applying the magnetic field to the sample. In alternatingmagnetic field, a frequency response of the magnetically induced strainswas measured at low frequencies. At higher frequencies, the frequencyresponse was measured using a strain gauge attached on the sample. Inthese measurements bar 2 was removed. This arrangement was also used inexperiments made on bent samples. the strain gauges were placed on bothsides of the bent sample. The changes of strains caused by the appliedmagnetic field were measured in static and alternating magnetic fields.

[0035] Arrangements for measuring electrical resistivity and magneticsusceptibility were also made on the sample holder, as shown in FIG. 6,namely the four point contacts 3 for resistivity and the coils 5 forsusceptibitity measurements. The sample chamber was immersed in liquidnitrogen or liquid helium, and the temperature could be controlledbetween 4 and 600 K using a heater 4.

[0036] Dissipation attributed to the motion of the twin boundaries andthe martensite interfaces was also studied with this equipment. Theamount of martensite was detected using electrical resistivity andmagnetic susceptibility measurements. Also Mössbauer spectroscopy wasused to determine the phase fraction of martensite. Mössbauerspectroscopy was more suitable for the present studies than X-rayspectrocopy, because Mössbauer measurements are not sensitive to thetexture of the sample.

Example 1

[0037] An alternating twisting deformation was applied to the sample,and the vibration damping capacity was measured. These experimentsrevealed that twin boundaries (as well as the interfaces betweenaustenite and the twinned martensite) were highly mobile. Themeasurements were made at strain amplitudes 10⁻⁶-10 ⁻³.

Example 2

[0038] Magnetically induced strains were measured on the bent samples.In the beginning, the martensitic sample was bent mechanically. Duringthis deformation one side of the sample was elongated and the other sidewas contracted. As a result, the twin structures on different sides ofthe samples were oriented in different ways, leading to the differentproportions of twin variants, to accommodate compressive and tensilestresses. It was confirmed that the amount of martensite was the same onboth sides of the sample. When the magnetic field was applied to thebent sample, the field-induced strains appeared, and they were inopposite directions on different sides of the sample. On the side whichwas initially elongated by the mechanical stress, the magnetic fieldinduced a contraction and on the other side the field induced anelongation. For example, when a magnetic field of 1 kOe was applied on aslightly bent twinned martensitic Fe—Ni—C sample of 1 mm in thickness,the difference in strains between the two sides was 2.2×10⁻⁵. This valueis higher than magnetostriction of this material. Magnetostrictioncannot be the explanation for this effect, because it cannot causestrains of the opposite directions in different sides of the sample, andsecondly because its magnitude is too small.

[0039] The initial mechanical deformation was also made by twisting. Thetwisting deformation results in a specific reorientation of the twinstructure. When the magnetic field was applied on this structure, thetorsional strains appeared.

[0040] The magnetically induced strains observed on the bent and twistedsamples could be attributed to the reorientation of the twin structureor the growth of the preferentially oriented martensite, The presentexperiments were made, however, above M_(d) temperatures of themartensite. At temperatures above M_(d), the formation of martensite isnot thermodynamically possible, which confirmes that the magneticallycontrolled reorientation of the twin structure causes the strainsobserved.

[0041] The bending and twisting experiments also suggest that morecomplicated shape changes can be produced using external magnetic field.

Example 3

[0042] X-ray diffraction patterns of martensite were measured inmagnetic fields perpendicular to and parallel to the surface of thesample. The intensities of the individual Bragg peaks correlate with thefraction of the twin variants in the diffraction condition. Themeasurements showed changes in peak intensities which were attributed tothe magnetically induced twin reorientation of the martensite. The peakintensities were observed to change also in alloys in which only theinner parts of martensite plates are twinned. The outer parts of theplates consist of dislocation cells and tangles. Therefore, theinterfaces between the martensite and austenite phases are immobile inthose alloys, and the magnetic-field-controlled growth of the martensiteplates with preferentially oriented twin variants cannot serve as anexplanation for the effects observed.

INDUSTIAL APPLICABILITY

[0043] The new actuators based on the present invention exhibit a greattechnological and commercial potential. No other method for producingmotion and force based on the material properties can develop such acombination of high strains, forces, speed and precision as these newactuators. Potential applications are fuel injectors, high-pressurepumps, actuators of active vibration control, active springs, valvelifters and controllers, robots, precision tooling and linear motors.Actuators can also be integrated with sensing and control capabilities.Those systems, named adaptive, active or smart structures, are becominggeneral in modem machine design. Sensing the operational parameters of amachine in real time, and responding to the environmental or internalchanges in a controlled manner makes it possible to attain more optimaloperation, minimal energy consumption, enhanced lifetimes of thestructures and lower maintenance costs. Adaptive structures are appliedin aerospace, automotive, marine and civil engineering, precisionmachining and production engineering. The most widely used actuators arepneumatic and hydraulic systems, electromagnetic drives and actuatormaterials such as piezoelectrics, magnetostrictive intermnetallics andshape memory alloys. Progress of the adaptive structures has beenseverely retarded by the absence of the high speed and large strokeactuator materials. The new materials based on the present invention maylead to a great advance in the technology of the adaptive structures andmodern engineering.

[0044] Because the reorientation of the twin structure occurs in threedimensions, complex shape changes including tension, bending andtwisting of the samples can be produced by the magnetic field. Thissignificantly expands the applicability of the present invention in manyfields of engineering and machinery. Other magnetically driven actuatorsbased on magnetostriction do not have such properties. Theactuator/machine developing a controlled motion or certain shape changesby the magnetic field may be an appropriately shaped and preorientedpiece of material. By designing the shape and the initial twin structureproperly, the actuator can repeat complex shape changes when theintensity of the magnetic field is cycled. The trace of the motion ofthe actuator can be changed by changing the direction of the field.

[0045] The method of the present invention makes it possible to controlthe operation of the actuators remotely. Remote control is suitable, forexample, in biomedical applications like in medical instruments,artificial organs, such as a heart. A large number of actuators couldoperate simultaneously using a common magnetic-field-control. Even ifthe magnetic field were the same for all of the actuators, the actuatorscould be made to operate in different ways depending on the initial twinstructure made in the material.

[0046] Because the twin structure is expected to be controlled also inthin films, wires and particles, the actuators based on this inventionmay also be applied in micro and nanotechnology. The actuators could beeven the size of the individual twins. The nanoactuators could utilize,e.g., quantum tunneling currents for the position sensing.

[0047] The present invention is a new method for producing motion, forceand shape changes using electric energy. Actuators based on this methodmay exhibit a potential to become the most widely used electric drivesafter motors and other devices based on electromagnetic forces. Inseveral fields of engineering, the new actuators are expected to replacethe conventional electric devices due to their better performance,greater reliability and lower costs. The largest potential of thepresent invention may lie, however, in new applications which only thetechnology based on this invention makes possible.

1. A method for controlling the orientation of the twin structure in amaterial having a twinned structure, comprising applying to the materiala magnetic field which is of the direction and of the magnitude enoughfor the reorientation of the twin structure of the material, to producethereby shape changes of the material and motion and/or force.
 2. Amethod according to claim 1, wherein the magnetic field is directed onthe material in the direction of the easy magnetization of the desiredtwin orientation.
 3. A method according to claim 1, wherein the magneticfield is directed on the material in such a direction that produces adesired shape change or motion of the material due to the reorientationof the twin structure.
 4. A method according to claim 1, wherein themagnetic field is directed on the material in the direction differingfrom the direction of the easy direction of magnetization of the twinvariants to produce bending or twisting of the material.
 5. A methodaccording to claim i, wherein the magnetic field is directed on thematerial in a changing direction and/or with changing magnitude as afunction of time.
 6. A method according to claim 1 wherein themagnetocrystalline anisotropy energy of the material is higher than orcomparable to the sum of the energies of the reorientation of the twinstructure required to produce a desired shape change and the work doneby the material.
 7. A method according to claim 1, wherein the energy ofthe magnetic field applied on the material is higher than or comparableto the sum of the energies of the reorientation of the twin structurerequired to produce a certain shape change and the work ofthe material.8. A method according to claim 1, wherein the material is ferromagnetic.9. A method according to claim 1, wherein the material is martensite.10. Use of the method according to any of the preceding claims to use anactuator wherein the shape change, motion and/or force of the actuatoris affected by the present method.
 11. Use of the method according toany of the preceding claims 1 to 9 to use actuators which arecontrolled, and the power of which is provided remote from theactuators.
 12. Use of the method according to any of the precedingclaims to use actuators in micro- and nanotechnology, in example theactuators made from twinned thin films, wires or particles.
 13. Use ofthe method according to claim 1, wherein the actuator is for pumps,injectors or similar devices for transferring material, especiallyfluid.